The present invention is directed towards the technology of signal transmission, and, specifically towards electronic circuits for power amplification.
Mobile communication devices, for example cellular telephones, need to transmit radio frequency (RF) signals at prescribed power levels. The prescribed power levels are dictated by the radio protocol standard under which the mobile device is operating. For example, in a GSM (Global System for Mobile communication) network, a mobile device transmits RF signals to base stations over a wide dynamic range, for example, up to 35 dBm, depending on the strength of the transmitted signal that the base station is receiving.
When transmission is initiated, the mobile device needs to reach the prescribed power level quickly, typically within about 30 microseconds. Further, the power level of the RF output signal must be maintained at the prescribed constant level.
To achieve the transmission power level, mobile devices include power amplifiers to amplify internally generated RF signals (RF input signal, or RF_IN) to RF transmission signals (RF output signal, or RF_OUT) having the prescribed power level. To maintain the power level of the RF output signal constant, a feedback control loop circuit is often used.
FIG. 1 (prior art) illustrates a simplified diagram of an RF amplifier circuit 20 having a known design. Here, a power amplifier 22 amplifies the RF input signal 21 and generates the RF output signal 23. To hold the power of the RF output signal 23 constant, a feedback control loop 25 is used. In feedback control loop 25, a resistor 24 connected to the power amplifier 22 is used to sample the current drawn by the power amplifier 22 by detecting the voltage drop across the resistor 24. The voltage drop is amplified and the amplified voltage is compared with a reference voltage, VREF. The reference voltage can be at the prescribed power level. The difference between the amplified voltage and the VREF is an automatic power control input voltage VAPC. The VAPC is then applied to the power amplifier 22. This negative feedback loop 25 causes the power amplifier 22 to draw a supply current that is proportional to the VREF. The power of the output signal 23 of the power amplifier 22 is proportional to the supply current. The supply current is drawn from a battery illustrated as the battery voltage VBAT in FIG. 1.
In this design, the amplifier circuit 20 has the disadvantage that the voltage drop across the resistor 24 reduces the voltage available to the power amplifier 22, thus lowering the power added efficiency of the amplifier circuit 20. In addition, the resistor 24 is a small low-tolerance resistor that can handle a high current. Such resistors tend to be expensive.
FIG. 2 (prior art) illustrates a simplified diagram of an RF amplifier circuit 30 again having a known design. Here, a power amplifier 32 amplifies the RF input signal 21 and generates an output signal 31 that is operated on by a feedback control circuit 35 before being directed to RF output node 33. In the feedback control circuit 35, a directional coupler 34 directs most of the output signal 31 to the RF output node 33, but a portion (xe2x80x9ccoupled signalxe2x80x9d) of the output signal 31 is directed to a detector 36. The detector 36 detects the peak voltage amplitude of the coupled signal and sends the detected voltage to an error amplifier 38. The detector 36 generally contains a detector diode and a reference diode. The error amplifier 38 compares the detected voltage, VDETECTED, to the VREF and generates an amplified automatic power control signal, VAPC, having a voltage proportional to the difference between VDETECTED and VREF. The output of the error amplifier 38 is applied to the power amplifier 32.
In this design, the amplifier circuit 30 has the disadvantage that the coupler 34 reduces the xe2x80x9cthrough outputxe2x80x9d power of the amplified output RF signal 31. The xe2x80x9ccoupled outputxe2x80x9d power is also reduced. Depending on the implementation, the reduction in through output power by the coupler 34 can be significant, thus decreasing the power added efficiency of the amplifier circuit 30. Further, the amplifier circuit 30 can be sensitive to temperature in its ability to operate properly. The correlation between the power of the output signal 31 and the VDETECTED can change with temperature variations. This is because, in a typical layout, the components of the amplifier circuit 30 are discrete components and that the components are not sufficiently proximal to each other within the amplifier circuit 20. In fact, components such as the coupler 34 are surface mount (SMT) components that take a relatively large area. Finally, the dynamic range of amplifier circuit 30 is limited. This is because the coupled signal is 10 to 13 dB lower than the output signal 31. As the power of the output signal 31 decreases, the voltage of the coupled signal can become insufficient to turn on the detector 36.
Accordingly, there remains a need for an improved power amplifier circuit.
These needs are met by the present invention. According to one aspect of the present invention, an amplifier circuit includes an amplifier amplifying an input voltage signal to generate an amplified voltage signal and a peak-to-peak detector connected to the amplifier detecting peak-to-peak voltage of the amplified signal to generate a detected signal. A comparator, connected to the amplifier and the peak-to-peak detector, compares the detected signal to a reference voltage signal, generating an automatic power control signal for controlling the amplifier.
According to another aspect of the present invention, an integrated circuit includes a negative peak detector receiving an input voltage signal and generating a first intermediate signal and a positive peak detector. The positive peak detector receives the first intermediate signal and generates a detected signal, the detected signal having value of peak-to-peak voltage of the input signal. The detected signal is substantially a direct current (DC) voltage.
According to yet another aspect of the present invention, a method of amplifying a signal is disclosed. First, an input RF signal is amplified, using an amplifier, to generate an amplified signal. Next, the peak-to-peak voltage of the amplified signal is detected using a peak-to-peak detector. The detected voltage is compared with a reference voltage to produce an automatic power control input signal. Finally, the automatic power control input signal is applied to the amplifier to control the degree of amplification.
Other embodiments and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.